Integrated circuit designers have always sought the ideal semiconductor memory: a device that is randomly accessible, can be written or read very quickly, is non-volatile, and consumes little power. Magnetoresistive random access memory (MRAM) technology has been increasingly viewed as offering all these advantages.
One form of magnetic memory element for an MRAM has a structure that includes ferromagnetic layers separated by a non-magnetic barrier layer that forms a tunnel junction. Information can be stored as a digital “1” or a “0” as directions of magnetization vectors in these ferromagnetic layers. Magnetic vectors in one ferromagnetic layer are magnetically fixed or pinned, while the magnetic vectors of the other ferromagnetic layer are not fixed so that the magnetization direction is free to switch between “parallel” and “antiparallel” states relative to the pinned layer. In response to parallel and antiparallel states, the magnetic memory element represents two different resistance states, which are read by the memory circuit as either a “1” or a “0.” It is the detection of these resistance states for the different magnetic orientations that allows the MRAM to read information.
Detection can be achieved by passing a current or bias voltage through the tunnel junction device in a direction perpendicular to the planes of the layers. The nonmagnetic insulating barrier layer that separates the pinned and free layers is thin enough that quantum mechanical tunneling occurs between the pinned and free layers. This tunneling is electron spin dependent, so that the directions of the magnetic moments of the pinned and free layers relative to each other affect the electron tunneling. When the magnetic orientation of the free layer changes, electron tunneling, and thus resistance, also changes. Specifically, when the relative orientations of the free and pinned layers become parallel, electron tunneling increases and the resistance decreases. When the orientations become antiparallel, electron tunneling decreases and the resistance increases. As the current is conducted through the element, the changes in resistance are manifested as potential changes and processed as a signal. The resistance change can also be expressed as a ratio of the change in resistance to the maximum resistance, or tunneling magnetoresistance (“TMR”) ratio.
One of the drawbacks of a memory element having a single magnetic tunnel junction as described above is a dramatic decrease in TMR with increased bias voltage. The decrease or decay in TMR results in a low cell signal and thus, difficulty in reading or sensing the state stored in the cell.
In addition, when a multilayered memory device is scaled down, numerous other problems can occur. For example, magnetostatic or dipolar coupling can occur between the pinned ferromagnetic layer and the free ferromagnetic, or sense layer. This coupling is due to the dipolar or stray fields of the ferromagnetic layers. When there is significant stray field from a pinned layer, the magnetostatic interaction between the pinned layer and free layer favors an antiparallel orientation in the layers' respective magnetic moments, resulting in an offset switch field at the free layer. In such a case, a hysteresis loop of the free or sense layer becomes offset from zero magnetic field. The offset field thus creates a situation in which a different write current is required to switch the magnetic moment in the sense layer from one state than is required to switch it from the other state. Write current would then need to be driven at the higher current at all times. This makes operation of the MTJ device difficult and leads to excessive power consumption. As these fields become larger with greater scale down, the problem exacerbates. In a more extreme but more important case, the bit cannot be written. Such a situation is illustrated in FIG. 2, in which the hysteresis loop indicates an offset 39. When the write field is set back to zero, the bit always magnetizes in direction 41 as opposed to direction 40.
Stray fields emanating from the free ferromagnetic layer also create operation difficulties in scale down. These stray fields interact with the free layers in neighboring bits arranged in an array, thus potentially causing inaccuracies in writing. Such fields therefore limit the density capabilities of an MRAM device.
Moreover, as lateral dimensions of the MTJ are reduced, the volume of each of the magnetic layers also decreases. This decrease in magnetic volume increases the possibility that “super-paramagnetic” behavior will occur. Super-paramagnetic behavior refers to a situation in which thermal fluctuations cause the magnetic moment of a magnetic entity to spontaneously rotate if the magnetic anisotropy of the entity, which is proportional to its volume, is not sufficiently great. Thus, thermal fluctuations can interfere with the operation or stability of the sense layer. As the super-paramagnetic limit is approached, data retention time is also reduced.
Devices having two tunnel junctions in a multiplanar orientation (“double tunnel junctions”), have been proposed to decrease the decay of TMR that occurs with increased bias voltage and to potentially increase the magnitude of the resistance change as the sense layer is reversed. In a double tunnel junction device, the decreased reduction in TMR with increasing bias voltage results because two barriers are biased in series. The voltage drop across each junction is effectively cut in half Likewise, the fact that two junctions are biased in series doubles the magnitude of the resistance change (not the percent change) between the low and high resistance states. Double tunnel junctions have been described which have two pinned layers pinned in the same direction, but such arrangements cause a large offset field at the sense layer. Other multilayer junction devices having one insulating layer and one conducting layer have been described in which two pinned ferromagnetic layers have fields pinned in opposite directions. Such devices cancel the offset field at the sense layer and may result in decreased power consumption but do not achieve substantially increased resistance change over other single tunnel junction devices.
There is thus a need in the art for a single MTJ memory element which mitigates the above disadvantages.